harden: keep the test recovery phrase out of release bundles, match committed keys by content (closes #351)

The 12-word BIP-39 test phrase survived in every release bundle as dead
text: module.exports keeps DEBUG_MNEMONIC live even though wallet.js's only
use of it folds away in a release build, so it could not be tree-shaken.
Putting the value itself behind the __BUILD_DEBUG__ define makes esbuild fold
it to null, so no distributed bundle carries it. script/verify-build now
fails a release build if the phrase appears in any emitted file, so the fold
cannot silently regress; test-verify-build covers both the release failure
and the debug allowance.

tests/extensionId.test.js now scans the content of every tracked file for a
PEM private-key header instead of matching filename extensions alone, so a
key committed under an unexpected name is caught.

Model: opus-4-8

tests/extensionId.test.js

@@ -89,19 +89,28 @@ describe("chrome extension identity", () => {
 
     // The private half is a credential. It has never been in this repo and no
     // target generates one into the working tree; this fails loudly if that
-    // ever changes, because a committed .pem is a key anyone can sign a CRX
-    // with under this extension's id.
+    // ever changes, because a committed private key is one anyone can sign a
+    // CRX with under this extension's id.
+    //
+    // Matched by CONTENT, not by filename: a key committed as notes.txt or with
+    // no extension carries the same risk as one named key.pem, and a
+    // filename-only check waves it through. The PEM header a private key opens
+    // with is the signature searched for. The pattern does not trip on its own
+    // source: the bracket-expression characters between the two anchors are not
+    // in the character class, so this file is not a match for it.
+    const PRIVATE_KEY_HEADER = /-----BEGIN [A-Z0-9 ]*PRIVATE KEY-----/;
     test("no private key is committed anywhere in the tree", () => {
+        const root = path.join(__dirname, "..");
         const tracked = require("child_process")
-            .execSync("git ls-files", {
-                cwd: path.join(__dirname, ".."),
-                encoding: "utf8",
-            })
+            .execSync("git ls-files", { cwd: root, encoding: "utf8" })
             .split("\n")
             .filter(Boolean);
-        expect(tracked.filter((f) => /\.(pem|key|p12|pfx)$/i.test(f))).toEqual(
-            [],
+        const offenders = tracked.filter((f) =>
+            PRIVATE_KEY_HEADER.test(
+                fs.readFileSync(path.join(root, f), "latin1"),
+            ),
         );
+        expect(offenders).toEqual([]);
     });
 });